1. Field of the Invention
The invention relates to an apparatus and method for trapping ions, in particular, a quadruple ion storage trap and method therefor.
2. Description of Related Art
Quadruple ion traps are characteristic of a family of instruments which includes a variety of mass spectrometers and mass filters. These types of instruments are used for mass analyzing and detecting electrically charged particles (ions) formed from atoms or molecules by extraction or attachment of electrons, protons or other charged species.
Ions, rather than neutral molecules, are analyzed because their motion is readily controllable in the gas phase using electric and magnetic fields. The main parameter which is used in these instruments for analyzing and separating the ions is the ratio of the mass of an ion to its charge (m/z). The mass (m) is usually expressed in atomic mass units (1 amu-1/12 of the mass of a carbon atom) and charge (z) is the number of charges of electron.
Ion traps are devices capable of storing one or more kinds of ions for long periods of time (from milliseconds to hours). This allows one to accumulate ions and study their properties and/or chemical reactions, in some cases, for example, by using external probes.
Ion traps also may be utilized as instruments for mass analyzing and detecting ions, and tandem experiments. When employing an ion trap, tandem experiments may be carried out in a single instrument, where successive reaction steps are separated in time, rather than space. Given the recognized success of conventional tandem instruments in structural biological research, this suggests broad opportunities for utilizing ion traps in biochemistry, protein chemistry and molecular biology to analyze the structures and sequences of biomolecules. Further, the ion storage capability of the ion trap allows one to carry out multiple fragmentation steps, and therefore has the potential for extending tandem (MS/MS) experiments to MS/MS/MS and beyond.
Conventionally, the second and third quadruple instruments that are commonly used for tandem experiments employ additional mass analyzers, resulting in a concomitant increase in expense. However, additional mass analyzers are not needed when a quadruple ion trap is employed.
The conventional quadruple ion trap was invented in the late 1950s by Wolfgang Paul from the University of Bonn (Paul, W.; Steinwedel, H.; German Patent 944,900,1956; U.S. Pat. No. 2,939,952, 7 Jun. 1960). Finnigan Corporation (Sunnyvale, Calif.) produced a commercial version of the quadruple ion trap known as the ion trap detector (ITD) which was used primarily as a low cost mass selective detector for gas chromatography.
The quadruple ion trap (ion trap) uses only time-varying electric field to trap ions and comprises a central, hyperbolic cross-section ring electrode positioned between two hyperbolic end-cap electrodes. This design and related operating characteristics of the Finnigan ion trap are shown schematically in FIGS. 1A and 1B. The RF electrode in FIG. 1A has rotational symmetry about a vertical axis.
Ions can be formed by variety of methods. In the Finnigan ion trap, ionization is performed by electron impact (EI). The ions are trapped and confined inside the ion trap cell by applying a radiofrequency (RF, usually approximately 1 MHz) voltage on the ring electrode with the end-cap electrodes being grounded. The time-varying quadruple electric field created in this configuration exerts forces on the ions which cause the ions to undergo vibrational motion about the center of the trap and then become "trapped" in the ion trap.
Ions of different m/z ratio can be trapped simultaneously. The mass range of ions that are trapped can be determined by the ion stability diagram and related equations shown in FIG. 2, using dimensionless parameters (a.sub.z and q.sub.z) that depend upon the radius of the trap (r), the DC (U) and RF (V) voltage amplitudes, and the RF frequency (w). The regions of stable motion in the vertical (z) direction (dark color) and in the plane of the ring electrode (grey color) are shown. The intersection of these two regions corresponds to stable trajectories in both directions. Ions not within this region collide with the walls of the trap and are lost due to neutralization.
By changing the operating parameters of the trap (i.e. U, V, or w) appropriately, it is possible to cause the ions to exit the trap in an order based on their mass/charge ratio. In this way, the ion trap can be utilized as a mass spectrometer to measure the molecular weights of the ions.
The most popular operational mode of an ion trap is the mass selective instability mode. In this mode, ions move along the q.sub.z axis (U=0) from the left to right side of the stability diagram with increasing RF voltage amplitude (V). Ions of increasingly higher mass arrive at the stability border in succession, exit the trap in the z direction and are detected by a multiplier located behind one of the end-caps (see e.g., FIG. 1A).
An important feature of a conventional ion trap is the presence of helium buffer gas within the trap at a relatively high (approximately 1 mtorr) pressure. A major function of the helium buffer gas is to decrease the ion kinetic energies through collisions, and to dampen the amplitude of ion motion thereby causing the ions to fall towards the center of the trap and remain there for a period of time. The stored ions can be expelled to the electron multiplier detector through a small perforation in the central part of the bottom end-cap electrode. The buffer gas also increases the mass resolution of the device when the scan speed is high. Alternatively, when lower scan speeds are used to increase mass resolution, the optimum pressure of the buffer gas also is lowered.
Finnigan later produced a more advanced version of the ion trap called the ion trap mass spectrometer (ITMS). The geometry of the ITMS was no different from that of the ion trap detector, but contained additional electronics, software, and the ability to provide supplementary RF voltages on the end caps, which were no longer grounded (see FIG. 3). This enabled one to control ion motion in the z direction via software control that was used for the more complex scan modes employed to perform MS/MS, etc. type experiments.
In recent years, considerable progress has been made in the development of the quadruple ion trap, primarily in mass range and resolution. The mass range has now been shown to exceed 70,000 daltons (Kaiser, R. E., Jr.; Cooks, R. G.; Stafford, G. C., Jr.; Syka, J. E. P.; Hemberger, P. H. Int. J. Mass Spectrom. Ion Processes, 106 (1991) 79). Mass resolution exceeding one part in 10.sup.6 has also been achieved (Williams, J. D.; Cox, K.; Morand, K. L.; Cooks, R. G.; Julian, R. K.; Kaiser, R. G. in Proceedings of the 39th ASMS Conference on Mass Spectrometry and Allied Topics, Nashville, Tenn., 1991, p. 1481). These achievements have become possible using the axial resonant ejection mode of operation and scan speeds slowed by a factor of 333 in comparison with those used in normal operation.
Axial excitation was first described in a patent (Syka, J. E. P.,; Louris, J. N.; Kelley, P. E.; Stafford, G. C.; Reynolds, W. E.; U.S. Pat. No. 4,736,101, 5 Apr. 1988) for a method intended to provide enhanced mass resolution. In this method, a bipolar, supplementary, low amplitude RF voltage is applied to the end-cap electrodes (see, for example, FIG. 3). The dipole electric field strongly affects the motion of ions of a particular mass/charge if the frequency of this field is in resonance with the frequency of their oscillation in z direction.
If the amplitude and duration of the supplementary RF excitation are small, then the ions exhibit an increase in their amplitude of oscillation, but continue to have stable trajectories. As the amplitude or duration of the excitation increases further, ions in resonance will exit the trap in the z-direction and be detected by the detector.
The resonant frequency depends upon the amplitude of the trapping RF field. One may obtain a mass spectrum by scanning the amplitude of the RF voltage applied to the ring electrode after the ions are trapped (see, for example, the RF voltage graph in FIG. 1B). Thus, the axial excitation scanning process is similar to that used for the mass selective instability mode of operation. However, in this case, ions can be ejected at any point along the q.sub.z axis lying within the stability diagram (see FIG. 2), while in the mass selective instability mode, they exit the trap along the extreme right point along the q.sub.z axis where it intersects the boundary of stability region (see FIG. 2). Thus, any mass may be ejected by the axial excitation method using an appropriate choice of the frequency of the exciting voltage.
During the past three years, considerable progress has been made in performing (MS).sup.n type experiments with the use of ion traps, with n&gt;8 achieved successfully. These achievements have been reviewed (March, R. E. Int. J. Mass Spectrom. Ion Processes, 118/119 (1992) 71), and have stimulated investigators in a number of fields to utilize this versatile ion trap instrument in their research. However, a major problem has been the ability to use the ion trap with ionization techniques that are capable of ionizing the large biomolecules (with molecular weights up to 10.sup.6 daltons) that are of interest to biochemists and molecular biologists.
Approaches for interacting ionization techniques with the ion trap can be divided between those which form ions directly inside the ion trap cell, and those which form ions in an external source and subsequently introduce the ions into the trap. A third method, developed in Johns Hopkins' laboratory (Heller, D. N.; Lys, I.; Cotter, R. J.; Uy, O. M., Anal. Chem., 61 (1989) 1083), involves forming ions on the inside surfaces of the trap. However, these ions may be considered to be formed by an external source because, in this method, it is necessary to overcome the same potential barrier for introducing and trapping ions in the center of the cell that exists for ions formed externally. That is, ions with low kinetic energies do not penetrate the potential barrier, while ions with higher kinetic energies will penetrate the potential barrier but not be trapped.
The most common example of internal ion production is the EI method used in the ion trap device developed as a mass selective detector for gas chromatography discussed previously. In the EI method, because the ions are formed in the gas phase in the center of the ion trap, the problems associated with introducing ions into the center of the trapping field does not exist. EI is one of the oldest ionization methods used in mass spectrometry, but it is not suitable for most modern applications because it requires that the neutral molecules to be ionized be volatile and thermally stable, and in addition causes excessive fragmentation.
For larger, non-volatile biomolecules, other ion forming methods have been developed and are known generally as desorption/ionization (DI) methods. Unfortunately, these ionization methods must be performed outside the trapping field. In these methods, ions are formed from surfaces which cannot be inserted directly into the electrostatic field because those surfaces would interfere with the field and the ion motion.
Desorption methods are utilized primarily for large molecules. The following desorption methods have been used with ion traps: secondary ion mass spectrometry (SIMS) in which secondary (sample) ions are desorbed from surfaces by a high energy beam of primary ions; fast-atom bombardment (FAB) where the primary particles are high energy neutral species; electrospray ionization (ESI) in which ions are evaporated from solutions; and laser desorption (LD) which utilizes a pulsed laser beam as the primary energy source.
Several years ago, a variation on laser desorption known as matrix-assisted laser desorption/ionization (MALDI) was developed. This method also has been used with ion traps. In that method, the biomolecules to be analyzed are recrystallized in a solid matrix of a low mass chromophore. Following absorption of the laser radiation by the matrix, ionization of the analyte molecules occurs as a result of desorption and subsequent charge exchange processes.
The ESI and MALDI methods have been the most prominent recently because they have been shown to be able to desorb intact molecular ions of proteins with molecular weights in excess of 100 kdaltons. Because the MALDI was the most recent method to be used with the ion trap, few reports describing that method exist (Doroshenko, V. M.; Cornish, T. J.; Cotter, R. J. Rapid Commun. Mass Spectrom., 6 (1992) 226) (Cox, K. A.; Williams, J. D.; Cooks, R. G.; Kaiser, R. E., Jr. Biological Mass Spectrom., 21 (1992) 226) (Chambers, D. M.; Goeringer, D. E.; McLuckey, S. A.; Glish, G. L. Anal. Chem., 65 (1993) 14-20).
As shown diagrammatically in FIG. 4, ions formed external to the ion trap may be introduced into the trapping field through a hole in the ring electrode or a hole in one of the end-caps, or through the space between the electrodes. As discussed previously, a major problem is that the ion kinetic energy needed to overcome the RF field also prevents trapping of the ions. There are two major approaches to overcoming this problem.
The first approach (Louris, J. N.; Amy, J. W.; Ridley, T. Y.; Cooks, R. G. Int. J. Mass Spectrum. Ion Processes, 88 (1989) 97) is to accelerate the ions to kinetic energies sufficient to overcome the potential barrier and introduce them into the active RF field, and subsequently reduce their kinetic energies through collisions with a buffer gas (helium). Since the amplitude of the RF field is constant during ion introduction, this method is most suitable for the continuous methods of ionization (SIMS, FAB, and ESI) where it has been actually applied. At the same time, this method has also been used in pulsed LD and MALDI configurations.
In general, this method is characterized by high pressures of buffer gas (more than 10.sup.-3 torr) and relatively low trapping efficiency, which is compensated by longer ion accumulation times when continuous ionization is utilized. Trapping efficiency decreases with the lower pressures required to achieve high mass resolution, and is inherently low for pulsed methods of ionization. These are main disadvantages associated with a method which carries out ion trapping in an active RF field.
The second approach involves gating the RF field synchronously with the introduction of ions into the trap. This was the subject of a patent (Dawson, P. H.; Whetten, N. R. U.S. Pat. No. 3,521,939; 1970), and described theoretically but never realized practically. (Kishore, M. N.; Ghosh, P. K. Int. J. Mass Spectrom. Ion Physics, 29 (1979) 345); (Todd, J. F. J.; Freer, D. A.; Waldren, R. M. Int. J. Mass Spectrom. Ion Physics, 36 (1980) 371); (O, C.-S.; Schuessler, H. A. Int. J. Mass Spectrom. Ion Physics, 40 (1981) 53); (O, C.-S.; Schuessler, H. A. Int. J. Mass Spectrom. Ion Physics, 40 (1981) 67); (O, C.-S.; Schuessler, H. A. Int. J. Mass Spectrom. Ion Physics, 40 (1981) 77).
In that approach, the RF field is off prior to introduction of the ions into the trap, and is turned on abruptly as the ions reach the center of the trap. Theoretical calculations suggested a relatively high efficiency for ion capture if the ion kinetic energy and the RF voltage amplitude are properly matched. A major problem with this method is that phase-synchronized switching of the RF amplitude demands that RF voltages of several kilovolts are turned on with a phase accuracy of at least 100 ns (for an RF frequency of 1 MHz). This dilemma has been the major obstacle to its implementation.
Another approach has been described in a publication (Sadat Kiai, S. M.; Andre, J.; Zerega, Y.; Brincourt, G.; Catella, R. Int. J. Mass Spectrom. Ion Processes, 107 (1991) 191). In that method, in place of the normal RF potential, V.sub.0 coswt, a periodic impulse potential of the form V.sub.0 coswt/(1-k cos2wt) where 0.ltorsim.k.ltorsim.1, was shown to be capable of trapping injected ions resulting from the presence of time-dependent zero potential zones.
Configurations for injecting ions from outside the ion trap differ somewhat from those used to desorb ions formed inside the trap near the electrode surface (as described by Heller et al. using infrared laser desorption). Typical designs for the first approach are shown in FIG. 5 for ESI at atmospheric pressure (Van Berkel, G. J.; Glish, G. L.; McLuckey, S. A. Anal. Chem., 62 (1990) 1284), and in FIG. 6 for LD ionization outside the ion trap (Mcintosh, A.; Donovan, T.; Brodbelt, J. in Proceedings of the 40th ASMS Conference on Mass Spectrometry and Allied Topics, May 31-Jun. 5, 1992, p. 1755).
As shown in FIG. 5, electrostatic lenses are used to focus the ion beam through the small centered hole in one of the end-caps. As shown in FIG. 6, the ions are formed near holes in the end-caps by laser radiation supplied through a fiber optic guide. No additional electrostatic optics were used in that method.
FIG. 7 shows the configuration used for forming ions at the inside surface of the ring electrode (see Heller, D. N.; Lys, I.; Cotter, R. J.; Uy, O. M. Anal. Chem., 61 (1989) 1083). In this system, two holes are drilled in the ring electrode to enable the sample probe and laser beam to be introduced at opposite directions.
Graphical representations of RF voltages applied to the ring electrode in prior methods for ion trapping are shown in FIGS. 8A and 8B. FIGS. 8A and 8B show an active continuous RF field and a synchronized switching RF field, respectively, that can be applied to the ring electrode. For a pulsed laser ionization source, some form of gating of the RF voltage is preferable. However, phase-synchronized, rapid switching of the RF high voltage (FIG. 8B) simultaneously with laser pulse is difficult to achieve and has been theoretically modeled but not implemented.
As known in conventional systems, several kilovolts are usually applied to the ring electrode to trap ions having energies of about 10 eV. Also, in the MALDI method, it has been shown that ions of different mass all have approximately the same velocities, so that their kinetic energies increase proportionally with the mass (Beavis, R. C.; Chait, B. T. Chem. Phys. Lett., 181 (1991) 497). Thus, high mass ions may have kinetic energies of 100 eV or more, requiring that an unrealistically high RF voltage be applied to the ring electrode to trap them in the ion trap.